Aerogels are materials consisting of a solid structure with very high porosity. Although they may consist of oxides of various metals or metalloids or mixtures thereof, the far most common and industrially important aerogels are the silica ones; in the present invention, therefore, reference is made to silica aerogels, but aerogels formed by mixed oxides may also be obtained by the methods described herein, containing silica as a main component and percentages of up to 45% of oxides of other metals, typically tri-, tetra- or pentavalent.
Silica aerogels are solids in which most of the volume, up to more than 99%, is occupied by gas (typically air), and only the remaining volume moiety consists of solid material; due to their structure, these materials can have a few milligrams per cm3 density and surface area values of between a few hundreds to about 1000 m2/g. Due to these features, aerogels are designed and used for some particular scientific applications (such as spatial source particle absorbers), as catalysts or catalyst supports, and mostly as thermal insulators due to their very low thermal conductivity (from 0.004 W/mK to 0.03 W/mK).
Silica aerogels are produced through processes called sol-gel.
There are numerous variants of sol-gel processes, which however have certain features in common. In these processes, one or more silicon compounds (defined precursors in the industry) are dissolved in water or water-alcohol mixtures, obtaining a solution called “sol”; the compounds present in the sol are then reacted, generally by destabilizing the system by changing the pH, resulting in a wet “gel”; the gel is then dried, according to various methods, forming a dry gel.
More specifically, in the aqueous or water-alcohol solution, a precursor undergoes an initial hydrolysis reaction in which one or more hydroxyl groups bind to silicon; formally, the reaction can be written as follows:

The species formed by hydrolysis of the precursor is normally defined as orthosilicic acid; in fact, as in the case of hydroxy compounds of other non-metals, it is an amphoteric species, whose formula can be written with the notation H4SiO4, respecting the formalism of acidic species, or with the notation Si(OH)4, more common in the field of sol-gels.
Orthosilicic acid has been observed only in highly diluted solutions since it is extremely unstable and spontaneously gives rise to the condensation reaction schematically represented below:

This reaction, repeated for all four —OH groups present on each silicon atom (polycondensation), leads to the formation of a three-dimensional pattern of Si—O—Si bonds and then to the oxide structure of the material.
The precursors used in sol-gel processes can be organometallic, such as the tetramethyl orthosilicate and tetraethyl orthosilicate compounds (of formula Si(OCH3)4 and Si(OC2H5)4, respectively, generally referred to as TMOS and TEOS); or inorganic, among which the most common ones are the alkali metal silicate solutions of general formula M2O×nSiO2 (M=Na, K, Li), wherein n is between 0.5 and 4; this general formula includes both stoichiometric compounds, such as sodium silicate, Na2SiO3 (n=1), and non-stoichiometric compositions. While the sol-gel processes starting from organometallic precursors are widely studied and used for scientific applications, the cost of these compounds makes them unsuitable for use in large scale applications.
The present invention is therefore directed to the production of aerogels starting from alkali metal silicate solutions, which can be produced starting from chemical compounds or as by-products of chemical processes, or from plant material containing large amounts of silicon, such as some by-products of rice processing.
The direct product of polycondensation is the wet “gel”, wherein the pattern of Si—O—Si bonds mentioned above forms an open structure that contains the solvent and reaction by-products in its porosities. The wet gel is usually subjected to washing step to eliminate the by-products (particularly if starting from inorganic precursors) and any soluble impurities, and/or exchange of the starting solvent with a different liquid to facilitate the subsequent drying operations.
The drying of wet gel can occur by simple evaporation of the liquid contained in the pores (thus obtaining dry gels called “xerogels”), or by extracting said liquid under supercritical conditions, resulting in the so-called “aerogels”.
While evaporation is simpler to practice, xerogels normally undergo significant reductions in volume compared to the starting wet gels (reaching volumes of about ⅛ compared to the volume of the wet gel) and extensive disruptions during the process, and they have a morphology, from the point of view of the pore distribution, completely different from the starting one.
On the other hand, hypercritical drying allows obtaining whole aerogels, in the industry referred as monolithic, which retain the shape and size of the starting wet gel but especially the pore morphology and distribution: this latter feature is required for some of the applications mentioned above, particularly for thermal, acoustic and electric insulation.
As said above, the most common silicate used in sol-gel processes is sodium silicate due to its low cost, wide availability, solubility in water and non-toxicity; in the remainder of the description, therefore, reference will be made to this compound obtained from vegetable matrices, but the invention is of general applicability starting from alkali metal silicates obtained by any manner.
Sodium silicate solutions have a strongly basic pH; the condensation of sols obtained from these solutions is generally obtained or accelerated by varying the pH value, bringing it from the starting values (about 13-14) to values generally between 4 and 10, by acid addition.
Processes of this type are described in several documents, including for example:                patent application CN 1449997 A, wherein HCl is added to a sodium silicate sol (which can have a concentration of between 0.01 and 1 kg/L) up to reach a pH of between 5 and 9;        the patent CN 1317188 C, wherein HCl is added to a sodium silicate sol (having a concentration of between 0.02 and 0.05 kg/L) up to reach a pH of between 6 and 8;        the U.S. Pat. No. 6,210,751 B1, wherein a sodium silicate sol with strongly basic pH is made to pass on an acidic ion exchange resin to remove sodium, or alternatively, an acid is poured in the silicate solution to then separate the resulting precipitate (Na2SO4), cooling the system to achieve an effective precipitation. In both cases, pH values of less than 4 are reached in the sol resulting from the treatment, to which a base (typically NaOH) is then added to bring the pH to a value of about 4.7;        the patent EP 1689676 B1, wherein rice husks are thermally treated at 700° C. until obtaining an ash, which is possibly prewashed with sulfuric acid; the ash is treated with NaOH, thus obtaining a sodium silicate sol, to which sulfuric acid is added, and after “aging” of the gel, it is washed with water to remove the resulting Na2SO4 salt; finally, the water in the gel is exchanged with an alcohol (typically ethanol) by means of a procedure with Soxhlet column, which is finally extracted under supercritical conditions;        the patent application WO 2005/044727 A1, wherein a solution containing Na2O and SiO2 in a molar ratio of between 1:3 and 1:4 and between 1 and 16% by weight of SiO2 is admixed with concentrated sulfuric acid (96% by weight solution; the final pH obtained is not indicated);        the article “Rice husk ash as a renewable source for the production of value added silica gel and its application: an overview”, R. Prasad et al., Bulletin of Chemical Reaction Engineering & Catalysis, 7 (1), 2012, 1-25;        the article “A simple process to prepare silica aerogel microparticles from rice husk ash”, R. S. Kumar et al., International Journal of Chemical Engineering and Applications, Vol. 4, No. Oct. 5, 2013;        and the article “Preparation of silica aerogel from rice hull ash by supercritical carbon dioxide drying”, Qi Tang et al., J. of Supercritical Fluids 35 (2005) 91-94.        
In these three articles, solutions of Na2O and SiO2 having a concentration of about 0.03 kg/L obtained by dissolving a precursor of SiO2 with NaOH having a concentration of 1 M is admixed with HCl, typically in turn having a concentration of 1 M, until a pH of between about 6 and 7 is obtained.
These known methods give rise to two types of problems.
Firstly, while adding the acid in the basic silicate solution, pH gradients are created which may lead to structural unevenness in the final gel.
Secondly and more importantly, during the gel formation step (beginning at about pH 10), this retains the alkali metals due to the slight acidity of silica within the porosity that is formed following polycondensation: these must be completely removed from the wet gel to prevent undesired consequences on the final aerogel, such as the tendency to become a dense glass already at relatively low temperatures (such as 600-700° C. in the case of sodium, depending on the content of the element).
The removal of alkaline and alkaline-earth elements from the wet gels is however a lengthy operation, given the very reduced size of the porosity of the same; in order to overcome this problem, it is known to subject the sodium silicate solutions, prior to gelling, to treatments with ion-exchange resins in order to replace the alkaline ion (e.g. Na+) with H+, or the separation of the salts formed by precipitation when adding acid. These operations increase the time required and complexity, and therefore the cost, of the overall process. Ion exchange treatments are described for example on page 50 (chapter 3, paragraph 3.2.1) of the book “Advances in Sol-Gel Derived Materials and Technologies”, edited by M. A. Aegerter and M. Prassas, and an example of these treatments for the removal of sodium is the process described in U.S. Pat. No. 6,210,751 B1.
Patent application CN 102757059 A follows a partly different method compared to the previous documents. The procedure is similar to that of patent EP 1689676 B1, but the sodium silicate solution is added to the acid one, controlling the addition so as to achieve a final pH of between 3 and 4. In order to effectively separate the salts precipitated from the gel, this is subjected to an electrophoretic treatment, introducing it into a container filled with water and applying an electric field to the system by two electrodes immersed in the water surrounding the gel, so that the positive ions are extracted from the gel and attracted towards the negative electrode. In addition to the process complication consisting of this further step, the present inventors have verified that it is very difficult to control the pH of the system to values of between 3 and 4, and that at these pH values, gelling occurs in a too early step of the process (approximately within two minutes from the mixing of the solutions), leading to an uneven system in which flocking gel fragments are observed within a still liquid phase.
The need is therefore still felt in the field to have a process for preparing silica aerogels starting from products having industrially acceptable costs and which is free from the drawbacks and complications of known processes.